Update: Dealer in Marbella Spain did "diagnostic" twice, first time told me the rear ASU air suspension units were likely faulty and needed changing. So I changed the side that was "sagging". No change. Second diagnostic, at 4! hours, and they told me the air line needed changing and 4! connectors. They wouldn't tell me exactly where the line was leaking and suggested "close to the ASU" Wanted $1,200 for the job too. So I spoke to Bentley UK tech, who told me they have a brass union that can be used to insert where the air line is damaged (No. 19 in the exploded picture of the air pump/module assembly). That cost me $25 plus freight. In my case it turned out the leak was close to the air control module / distribution bloc, (so the OTHER END of what the dealer told me), where another part had filed through the plastic line via simple wear and tear (unbelievably the lines are unprotected and there is arguably a design fault in this area under the car (beneath the spare wheel well). Anyway, cut the line and inserted the brass union. And so far so good - the car is now sitting as it should.
With hindsight, I should have inspected the assembly and the line. The leak was fairly obvious once the under-chassis cover was off (itself very simple).

Moral of the story is, I suppose, sometimes the ASU fails, sometimes it is the air line or something else. Some dealers seem to make a beeline to the ASU exchange which is a very expensive parts repair, in my case without actually checking if that is the real fault.

Thanks vm for the pointers along the way, especially Johnny. Had I avoided the dealer, and followed JohnnyJJ's input from day one I would have saved a bundle..

Hi Lars.

At least it's sorted and lesson learned.
Might take a look at mine just to check for potential rubbing on the hoses.

Les.

Cheers Les,

That is a good idea to do. The blue line going to the rear right side is near to or touching a hard-plastic electrical plug near the distributor module, and in my case that had "sawed" through the plastic air line over time..

The lines should have been protected better..

Lars

....and so it goes, Diogenes seeking an honest Diagnostician...this is the principle reason I have arranged to leave the Bentley to a person other than my wife....